The present invention relates to an apparatus and method for welding pipes together. In particular, but not exclusively, the invention relates to welding together clad pipe sections, or other multilayer pipe sections, when laying pipelines underwater at sea.
When laying a pipeline at sea from a pipe-laying vessel, one end of the pipeline (sometimes referred to as the “pipe string”) is held by the vessel and a pipe section is welded to the end of the pipeline. The pipeline and newly welded pipe section are then fed out from the vessel, so that the process can be repeated. The weld joint must be of high quality in view of the high tensile loads to which the pipeline is subjected during the laying process. Quality of weld joints is critical when the pipeline will, once installed, be in a position where it is exposed to regular changing motion in the sea (for example when the pipeline is in the form of a catenary riser). In such case, the weld joint must be able to withstand the fatigue loads to which it will be subjected.
Pipelines designed to convey liquid or gases that are corrosive or otherwise able to damage normal steel pipes are typically internally clad or otherwise lined with CRA (corrosion resistant alloy). Also, such CRA-lined pipelines are increasingly found to offer an economic solution to the exploitation of remote wells. CRA-lined pipes are made by metallurgically bonding, or pressing, a CRA cylinder to the inside of a carbon steel (CS) pipe.
The process of welding together CRA-lined pipes is more complex than welding of normal CS pipeline. It should be noted that the production rate for a CRA pipeline is typically a factor of 4 or 5 times less than that of a CS pipeline of the same dimensions. A typical CS pipe clad with CRA materials and the weld joint formed therebetween is shown in FIGS. 1a, 1b and 2.
A known technique for welding together CRA-lined pipe sections is to use a single externally mounted semi-automated GMAW welding head for depositing the critical first pass indicated as root pass, followed by an internal inspection to ensure the quality of the root weld is of a sufficiently high standard. The weld area is purged with protective shielding gas (usually an inert gas such as argon) to reduce the risk of oxidation of the weld material. The internal inspection includes an ultrasonic testing regime and a visual inspection performed with the aid of cameras. The visual inspection is required because the ultrasonic testing can be prone to false results due to residual magnetization of the CRA material.
As a result of the nature of the root-welding process, it is difficult to ensure proper weld penetration. The whole welding process is very sensitive to variations in many parameters such as joint fit-up, levels of magnetism and even small changes in gas composition. Despite the large degree of automation that such a welding process provides, the acceptance of the weld still remains very dependent on the skill of the welder. The overall cycle time to complete the first two root passes and perform the internal inspection is relatively extended. Furthermore, if the root weld fails to meet the strict acceptance criteria, it is generally the case that root repairs are not performed and instead the whole weld-joint is cut-out, leading to further delays. The joint is welded to the hot pass stage and internally inspected before being moved out of the welding station. When steady-state production is being achieved, a welding cycle time of about 30 minutes at the first welding station may be achieved, but achieving a welding cycle of that duration can be a challenge when laying pipeline at sea. It will be appreciated that after the root weld is finished, the pipe is moved to subsequent stations to be filled with the other weld passes and/or processed/tested in other ways. The operations at the station where the root pass is laid down tend however to be the rate-limiting step.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method of laying a pipeline, an improved welding method, and/or an improved pipe-welding apparatus.